Titanium-Based Alloy

ABSTRACT

The titanium-based alloy consists of aluminum, vanadium, molybdenum, iron, and oxygen in the following weight percent ratio: aluminum 3.5-4.4, vanadium 2.0-4.0, molybdenum 0.1-0.8, iron maximum 0.4, oxygen maximum 0.25, the balance titanium. The technical objective is to provide a versatile alloy to be used for making large forgings and die forgings, rolled sheet products and foil having sufficient strength, ductility and structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of metallurgy and particularly to the field of developing state-of-the-art titanium alloys used for making high-strength and high-workability articles including large articles, i.e., alloys of high versatility.

Titanium alloys are widely used as aerospace materials, e.g., in air-planes and rockets, since the alloys are mechanically tough and are comparatively light.

2. Background Information

The most widely used titanium alloy is Ti6Al4V (B. A. Kalachyov, I. S. Polkin and V. D. Talalayev. Titanium Alloys of Different Countries. Reference Book. Moscow: VILS, 2000, p. 58-59-[1]). This alloy was developed in the USA during the 1950s. It is characterized by medium strength of 850 up to 1000 MPa and high workability. It is a good material to work by forming: forging, die forging, and extruding. It is widely used in aeronautical and aerospace engineering, shipbuilding, the automotive industry, etc., as well as in manufacturing fasteners for various applications. This alloy is good for working by all types of welding including diffusion bonding.

The disadvantage of Ti6Al4V is its insufficient versatility. It is difficult to make rolled sheet products, foil, and tubes thereof since the alloy possesses relatively high resistance to deformation, which, at deformation temperatures below 800° C., leads to the generation of defects such as cracks and shortens the life of working tools, or necessitates costly tools.

Pseudo-α-titanium alloy Grade 9 (Ti-3AI-2.5V) is highly cold-workable (see [1], p. 44, 45). The strength of this alloy is intermediate between that of Ti-6AI-4V and that of titanium (600-800 MPa). This alloy is used cold-worked and stress-annealed; it is characterized by high corrosion resistance in various media including sea water. This alloy is used in making tubes for hydraulics and fuel systems of airplanes, rockets, and submarines.

The disadvantage of this alloy also is its low versatility since it requires stress relieving in making large structural parts thereof. Therefore, such articles have to be annealed which reduces the strength of the Grade 9 alloy down to 400-500 MPa.

The closest analog of the invented alloy is an α+β-titanium alloy consisting of 3.0-5.0 Al; 2.1-3.7 V; 0.85-3.15 Mo; 0.85-3.15 Fe; 0.06-0.2 O₂, and inevitable impurities (prior art Japanese application No. 3007214 B2, filed Feb. 7, 2000).

The disadvantage of this alloy is that it is rich in Fe and Mo and, therefore, is prone to segregation. In order to reduce the possibility of segregational heterogeneity a special ingot melting technology must be used, followed by rolling and forging at a small deformation rate in order to exclude decoration by “beta-flecks”, which processing decreases productivity.

OBJECT OF THE INVENTION

It is an object of the invention to provide a versatile titanium alloy having minimal manufacturing costs and capable of being made into a wide variety of products, such as large forgings and die forgings, as well as rolled sheet products and foil having sufficient strength, plastic properties and structure.

SUMMARY OF THE INVENTION

According to the invention an optimum mix of α- and β-stabilizing alloying elements is provided in a semi-finished product.

The invention provides a titanium-based alloy consisting of aluminum, vanadium, molybdenum, iron, and oxygen in the following weight percent ratio:

wt. % aluminum 3.5-4.4 vanadium 2.0-4.0 molybdenum 0.1-0.8 iron max 0.4  oxygen max 0.25 titanium balance

The combination of high strength and ductility in the invented alloy is achieved through targeted selection and experimental evaluation of the alloying ranges. The content of α-stabilizers (aluminum, oxygen) and β-stabilizers (vanadium, molybdenum, and iron) was determined so as to meet the objective.

Aluminum is an α-stabilizer for the α+β-titanium alloys, which contributes to increased mechanical strength. However, if the aluminum content is below 3.5%, strength sufficient to meet the invention goal cannot be obtained; whereas if the aluminum content exceeds 4.4%, resistance to hot deformation is increased and ductility at lower temperatures is decreased, which leads to lower productivity.

Vanadium is added to titanium as a β-stabilizer for the α+β-titanium alloys, increasing mechanical strength without forming brittle intermetallic compounds with titanium. The presence of vanadium in the alloy impedes formation of α₂-superstructure in the α-phase as the β-phase stabilizes, and increases both strength and ductility. If the vanadium content is below 2%, strength sufficient to meet the invention goal cannot be obtained; whereas if the vanadium content exceeds 4.0%, the superplastic elongation is decreased by lowering of the beta transus. Vanadium content within the range of 2.0-4.0% in this alloy has the benefit that scrap of the most-used Ti6Al4V can be utilized.

Molybdenum is added to titanium as a β-stabilizer for the α+β-titanium alloys. If molybdenum is added within the range of 0.1-0.8%, it can fully dissolve in the α-phase, so that sufficient strength is obtained without deteriorating plastic properties. If the molybdenum content exceeds 0.8%, it increases the specific weight of the alloy due to the fact that molybdenum is a heavy metal, and the plastic properties of the alloy deteriorate. If the molybdenum content is below 0.1%, molybdenum does not fully contribute to the alloy's properties.

Iron added to the alloy up to 0.4% increases the volume ratio of the β-phase, decreasing resistance to deformation in hot working of this alloy, thus evading the generation of such defects as cracking. An iron content exceeding 0.4% generates a segregation phase with beta-flecks upon melting and solidifying the alloy, which leads to heterogeneity of mechanical properties, especially ductility.

Oxygen enhances mechanical strength by constituting a solid solution, mainly in the α-phase. If the oxygen content exceeds 0.25%, the alloy ductility may deteriorate.

The alloy may contain up to 0.1% of carbon and up to 0.05% of nitrogen as inevitable impurities; the total quantity of impurities shall not exceed 0.16%.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

To evaluate the properties of the claimed alloy ingots were melted by the method of double vacuum-arc remelt, having the following chemical compositions (Table 1).

TABLE 1 Chemical Composition, wt. % Alloy Al V Mo Fe O 1 3.9 2.2 0.2 0.13 0.17 2 4.3 2.8 0.3 0.24 0.23 3 4.3 3.3 0.6 0.32 0.20

TABLE 2 Mechanical Properties σ_(?), σ_(0.2), Alloy Heat Treatment MPa MPa δ, % ψ, % 1 W/o annealing 810 735 15.2 38.2 750° C. 1 hour, air 780 693 13.2 32.0 2 W/o annealing 960 840 14.2 33.1 750° C. 1 hour, air 920 845 13.6 32.5 3 α + β 710° C. 3 hours, 900 835 15 33.0 air β 710° C. 3 hours, 870 800 14 28.0 air

A bar of 50-mm diameter was made of each ingot by hot working. Part of the bars were heat treated by annealing at 750° C., soaking for 1 hour and cooling in the air. The mechanical properties at room temperature were evaluated for the bars heat treated and for those not heat treated. The evaluation results are given in Table 2. In addition, the mechanical properties of upset β-phase workpieces, which were heat treated at 710° C., soaked for 3 hours and cooled in air, were evaluated. The results of mechanical testing of upset α+β and β-field workpieces are given in Table 2.

In comparison with known alloys the invented alloy is highly versatile, economically beneficial and has lower cost due to the fact that scrap of widely known alloys, such as Ti6Al4V, can be used for its production. This alloy possesses required and sufficient mechanical properties and can be utilized for making a wide range of products, such as large forgings and die forgings, thin sheets and foil, by working in both the α+β-field and the β-field. 

1. A titanium-based alloy substantially consisting of aluminum at 3.5 to 4.4 wt. %, vanadium at 2.0 to 4.0 wt. %, molybdenum at 0.1 to 0.8 wt. %, iron at a maximum of 0.4 wt. %, oxygen at a maximum of 0.25 wt. % and a balance of titanium and inevitable impurities. 